Hitherto many proposals with respect to a process for preparing a granular powder by pulverizing a PTFE powder prepared through suspension polymerization to an average particle size of less than 100 .mu.m and then granulating have been made. For example, JP-B-44-22619 discloses a process for stirring and granulating a PTFE powder in an aqueous medium of 30.degree. to 150.degree. C. containing a water-insoluble organic liquid having a boiling point of 30.degree. to 150.degree. C.
In order to improve the process disclosed in the above-mentioned patent publication, JP-B-57-15128 discloses a process in which an equipment having mechanism for pulverizing a PTFE powder is employed.
As a process for granulating by using water only, for example, JP-B-43-8611 discloses a process for granulating in which a PTFE powder is stirred in water of 40.degree. to 90.degree. C., JP-B-47-3187 discloses a process for granulating in which a PTFE powder is stirred in an. aqueous medium of not less than 40.degree. C., and further JP-A-3-259926 discloses a process for granulating in which a PTFE powder is stirred in an aqueous medium in combination use of pulverizing mechanism.
However, in such granulation processes, electrostatic charging occurs on the PTFE powder due to static electricity. The electrostatically charged PTFE powder adheres to not only a die mold but also a hopper and feeder due to static electricity at the time of molding, which results in lowering of flowability. Also, there is a tendency that the finer the powder is, the more the flowability and apparent density are lowered. From these points of view, the PTFE powder obtained by the above-mentioned processes is not satisfactory.
Also a granulation process with a roughly pulverized PTFE powder having an average particle size of not less than 100 .mu.m has been proposed. As the process using a roughly pulverized powder, for example, JP-A-3-259925 discloses a process for granulating in which a PTFE powder of 420 .mu.m particle size is stirred in an aqueous medium of 60.degree. to 100.degree. C. in combination use of pulverizing mechanism.
Even by the process disclosed in the above-mentioned patent publication, the obtained PTFE granular powder does not sufficiently satisfy physical properties of a molded article such as tensile strength, and for commercial production, a further step such as a gelling and pulverizing step is required. Also since a molded article obtained by molding the granular powder has a low break down voltage, it cannot be used for applications requiring a high break down voltage. Further as mentioned above, that process requires the pulverizing mechanism.
Also JP-B-3-39105 discloses a technique for preparing a granular powder by granulating a modified PTFE powder with stirring in hot water under coexistence of an organic liquid. With respect to the granular powder prepared by that method and having an apparent density of at least 700 g/liter (0.7 g/cm.sup.3), when the powder has an average particle size of 150 to 250 .mu.m, flowability f thereof is from 8.0 to 4.0 sec/50 g and particle stability s is from 8.0 to 4.0 sec/50 g, and when the powder has an average particle size of 250 to 1000 .mu.m, flowability f thereof is from 4.0 to 1.5 sec/50 g and particle stability s is from 4.0 to 1.8 sec/50 g. Also it is disclosed therein that with respect to the granular powder having an apparent density of at least 800 g/liter (0.8 g/cm.sup.3), when the powder has an average particle size of 150 to 250 .mu.m, flowability f thereof is from 6.0 to 4.0 sec/50 g and particle stability s is from 6.0 to 4.0 sec/90 g, and when the powder has an average particle size of 250 to 1000 .mu.m, flowability f thereof is from 3.0 to 1.5 sec/50 g and particle stability s is from 3.0 to 1.8 sec/50 g. The particle stability means flowability of a powder after a stress is applied mechanically to the powder, and is used for evaluating flowability after storing or transporting particles. Methods for measuring the flowability and particle stability are described hereinafter.
The modified PTFE granular powder disclosed in JP-B-3-39105 is surely improved in that the number of pores (measured as electrical defects) on a 200 .mu.m thick sliced film obtained from an agglomerated molding powder is small under a voltage of 5000 V. However flowability is wholly insufficient (angle of repose: not less than 39 degrees). Also a relatively high flowability range (high apparent density and large particle size), a degree of lowering of particle stability, namely a particle stability coefficient (particle stability s/flowability f) is large (In Example 13, the particle stability coefficient 2.5/1.9=1.32).
Also that granular powder has high electrostatic charge of not less than 300 V. A break down voltage of a molded article obtained by molding the powder is as low as not more than 10 kV and a surface roughness is as rough as not less than 3.0 .mu.m.
The present inventors have made intensive studies in view of the above-mentioned problems, and as a result, have found that those problems can be solved by granulating a modified PTFE powder in the presence of an organic liquid which forms liquid-liquid interface with water, and a specific nonionic surfactant with stirring in water.
Namely, an object of the present invention is to provide a modified PTFE granular powder being excellent in powder characteristics such as powder flowability, particularly particle stability and having a large apparent density, a small average particle size, a narrow particle size distribution, a small electrostatic charge (substantially approximate to zero) and a small angle of repose. The modified PTFE granular powder gives a molded article being excellent in physical properties such as elongation and surface smoothness and having a small surface roughness (smooth) and a large break down voltage.